There is a growing need to supply fresh water on a global basis to meet the needs of expanding populations. A variety of membrane technologies are actively employed to meet this need. Microfiltration (MF) and ultrafiltration (UF) are used to purify surface waters for drinking, pre-treat brackish and seawater for reverse osmosis, and treat wastewater (especially in membrane bioreactors) prior to discharge into the environment.
Polyvinylidene fluoride (PVDF) is a preferred polymer material for MF and UF membranes due to its excellent chemical resistance, especially to oxidants and halogens used in water purification. PVDF is also convenient to process by solution casting (or melt casting) into porous membranes. While PVDF is well established in microfiltration (nominal pore size >0.2 um) it has difficulties in smaller pore size membranes due to lower water flux. As pure water regulations become increasingly stringent, there is a move to require microfiltration membranes to filter below 0.1 um for removal of virus particles. This is now in the ultrafiltration range. For PVDF to work well in this smaller pore regime, it will be necessary to improve membrane water flux.
PVDF is a very hydrophobic polymer, which increases resistance to water flux in membranes. Making PVDF membranes more hydrophilic is essential to improving water flux. Many methods have been described for post treatment of PVDF membranes. These methods typically involved treating a porous PVDF membrane with a hydrophilic monomer (e.g. acrylic acid or hydroxyethylmethacrylate) followed by polymerization to create a hydrophilic surface treatment. These methods are described in U.S. Pat. No. 4,618,533 (M. J. Steuck inventor, Oct. 21, 1986), U.S. Pat. No. 4,855,163 (I. B. Joffe, P. J. Degen, F. A. Baltusis inventors, Aug. 8, 1989), and R. Revanur et al Macromolecules 2007, 40, 3624-3620. These polymerizations are typically free radical in nature, initiated either chemically or by radiation.
Other post treatment methods include soaking membranes in hydrophilic polymer solutions (e.g. hydroxyethylcellulose, polyethylene glycol) to impart a temporary hydrophilic coating, as described in U.S. Pat. No. 4,203,848 (J. D. Grandine II inventor, May 20, 1980). This method is suitable for single use, or batch process applications, but will not retain hydrophilicity for extended service. Other treatments involve chemical etching of the PVDF surface (by either caustic treatment or radiation), with or without, subsequent by oxidative treatment to create permanent hydrophilicity.
These post treatment methods add complexity and cost to the membrane manufacturing process. They require separate washing and drying steps to remove all post-treatment chemicals. These treatments may also be limited to surface functionality, while leaving internal pores hydrophobic. Alternatively, excessive polymerization of added monomers can lead to pore plugging or pore size reduction. This will reduce membrane flux. Finally, radiation and surface grafting processes will physically damage the PVDF polymer backbone, reducing mechanical properties.
Furthermore, capillary hollow fiber membranes are difficult to post treat by the above processes, and are even more susceptible to physical damage. Yet capillary hollow fiber membranes are the preferred choice for water treatment systems due to superior volumetric system performance. Improving hydrophilicity of PVDF hollow fiber membranes is a major technical challenge that remains to be solved.
Amphiphilic block copolymers are well known. The majority of amphiphilic polymers are diblock copolymers that are soluble in water. These diblock polymers have been used for a number of applications including, the thickening of aqueous solutions and to form viscoelastic gels, such as those described in U.S. Pat. Nos. 6,506,837, 6,437,040, and U.S. Patent application 2003/0162896.
Other amphiphilic triblock copolymers, known commercially as Pluronics, are also well described in the literature. These triblock copolymers can contain hydrophilic endblocks and a hydrophobic midblock or vice versa. The hydrophilic block segment is confined to polyethylene oxide (PEO) homopolymer. The triblocks containing hydrophilic endblocks are soluble in water. The triblocks containing hydrophobic endblocks will be insoluble in water.
Amphiphilic diblock polymers may be formed using stable free radical chemistry, as described in U.S. Pat. No. 6,111,025. The polymers described are limited to diblock structures, and furthermore describe the use of TEMPO-based nitroxide derivatives for he synthesis of the corresponding block copolymers. This class of free radical control agent [such as (2′,2′,6′,6′-tetramethyl-1′-piperidyloxy-)methylbenzene mentioned in Macromolecules 1996, 29, pages 5245-5254] control only the polymerizations of styrene and styrenic derivatives and are not suited to the controlled polymerization of acrylics. U.S. Pat. No. 6,767,968 describes the use of living-type or semi-living type free radical polymerization to form ABA triblock copolymers having a random block with both hydrophobic and hydrophilic monomer units.
Arkema patent application U.S. 2006052545 describes a diblock copolymer adhesive formed by a controlled radical polymerization that is capable of absorbing water and providing adhesion under humid conditions.
Amphiphilic block polymers for use as additives and thickeners in oil-based compositions are described in WO 05/056739. U.S. 2008-0058475 describes the formation of amphiphilic triblock copolymers for use as hydrogels.
None of the aforementioned references teach the use of amphiphilic structures for membrane modification.
Specially grafted PVDF polymers have been tested in membrane applications. Grafting was accomplished by a controlled radical polymerization process (ATRP) to build hydrophilic groups onto the bulk polymer surface. This grafting technique is described in PCT WO 02/22712 A2 (A. M. Mayes, J. F. Hester, P. Banerjee, and A. Akthakul inventors, Mar. 21, 2002). This grafted polymer can then be fabricated into porous membranes using the phase inversion process. As with the other treatments, this approach adds significant cost to the process, as it is a multi-step process with several post polymerization transformation steps. Chemical modification of the bulk PVDF polymer can lead to crosslinking, which may affect solution properties, and ultimately the porosity of membranes. Therefore, direct modification of PVDF is a very costly and potentially unreliable means to make hydrophilic membranes.
Mayes and coworkers reported on blends of random acrylic-copolymers with PVDF to improve hydrophilicity Macromolecules, 1999, 32, 1643. These acrylic copolymers contained hydrophilic monomer groups such as polyethylene glycol methacrylate. As random copolymers, it is more difficult to control microstructure and composition than with controlled radical polymerization. It is argued that these hydrophilic “comb” polymers were specifically designed to surface segregate and concentrate at the membrane surface due to thermodynamics based on the theory that highly branched structures tend to surface segregate. While this is effective to improve surface hydrophilicity, there is a question as to how effective this treatment is for uniform hydrophilicity within the membrane pores. Higher surface concentration may also promote loss of additive by leaching, i.e., the hydrophobic backbone can't efficiently entangle with the polymer matrix. Furthermore, the heterogeneous structure of the polymers formed from the free radical process may limit the efficiency. It is well known that the control of architecture and chemical composition of both the backbone and the grafts are much more difficult to achieve as compared to block copolymers. Traditional free radical polymerization does not allow for the specific tailoring of polymer segment control and property adjustment.
Surprisingly, it has now been found that the addition of controlled architecture amphiphilic block copolymers to a hydrophobic polymer matrix in a polymeric membrane provides a stable pore size and increased flux, even at very small pore sizes. The block copolymer blends described herein are specifically designed to impart uniform and stable hydrophilicity. A fluoropolymer matrix with acrylic amphiphilic block copolymers forms a stable blend that is easily cast into membranes having a high water flux. Unlike conventional hydrophilic additives, the properly tailored block copolymers do not leach out with prolonged water washing. The addition of the amphiphilic block copolymers are especially useful in microfiltration and ultra filtration membranes when used in water filtration.